12 Digestive system

The digestive tract is a long muscular tube lined with epithelium specialized for digestion and absorption of food and water. Food moves along the digestive system from the mouth where it is ingested, to the anus where the undigested and unabsorbed remnants of food and some additional waste are eliminated.

The digestive tract (a.k.a alimentary tract), starts in the oral cavity and continues through the pharynx, to the esophagus, stomach, duodenum, small intestine, large intestine, rectum, and terminates in the anal canal. Food moves along the digestive tract by peristalsis, the rhythmic contractions of the smooth muscle within the walls of the tube. Food moves in one direction (except in unusual circumstances such as vomiting), and multiple circular muscles called sphincters, located at critical junctions, prevent the food in transit from going backward. The mucosal (epithelial layer) secretions aid in digestion, and later provide the mechanisms for the absorption of nutrients.  

In addition to the digestive tract, the digestive system includes several accessory glands that secrete various enzymes and fluids to assist with digestion and transport. The major accessory glands of the digestive system include:

  • three pairs of salivary glands,
  • the pancreas
  • the liver with the gallbladder.
Figure 1: Segments and accessory glands of the digestive system.

 

The structure of the intestinal wall changes along the digestive tract, reflecting the function of the particular segment, but the general architecture remains the same.

Four layers of digestive tract walls

Walls of the digestive tract have four concentric layers. Going from the inside out, these are:

    • mucosa
    • submucosa
    • muscularis externa
    • adventitia or serosa
Figure 2: Schematic drawing of the digestive tract layers

Mucosa

The mucosa is the innermost layer. It is made of:

    • epithelium
    • lamina propria
    • muscularis mucosae (this is separate from the thicker layer of smooth muscle within the muscularis layer).

Epithelium covers the inner surface of the digestive tract. It starts as stratified squamous epithelium in the esophagus and changes to simple columnar epithelium in the stomach. In the intestines, it stays columnar but acquires microvilli to increase the surface area for absorption. The segmental characteristics of the epithelium will be described in later sections. As with every other epithelium, it lies on the basement membrane.

The lamina propria, a thin layer of loose connective tissue, lies directly below the mucosal epithelium.

The muscularis mucosae is a relatively thin layer of smooth muscle located between the mucosa and the submucosa.

Submucosa

The submucosa is composed of a layer of dense, irregular connective tissue. It contains large blood vessels, lymphatics and the neurons of the submucosal plexus of Meissner.

Muscularis externa

The muscularis, sometimes called muscularis externa to differentiate from the muscularis mucosae, consists of two clearly visible layers of smooth muscle (three in the stomach only):

    • an inner circular layer
    • an outer longitudinal layer

Another nerve plexus, a myenteric plexus of Auerbach, lies between the circular and longitudinal layers of smooth muscle. This muscular layer contracts to produce peristalsis.

Adventitia

The adventitia is the outermost layer and is a thin layer of loose connective tissue. In places, a thin layer of simple squamous epithelium called mesothelium covers adventitia on the external surface. When covered by mesothelium, the adventitia is called the serosa.

Regional histology of the digestive tract. Courtesy of Andrea Campo-Velez.

 

Figure 3: Layers of the digestive tract walls at low magnification. The image is of the esophagus.
Figure 4: Layers of the digestive tract walls at higher magnification. 1. mucosa 2. submucosa 3. muscularis externa 4. adventitia. The image is of the esophagus from above.

Regional histology of the digestive tract

Esophagus

The esophagus is a long, soft tube that connects the pharynx to the stomach. Its only function is the transport of food.

The mucosal surface of the esophagus is lined by a thick layer of stratified squamous epithelium, adapted for fast transport and withstanding abrasive forces of moving food pieces. The muscularis is well developed as the esophagus’ peristalsis has to push food toward the stomach. In the upper segment of the esophagus, the muscularis layer contains mostly skeletal muscle that transitions to a mixture of skeletal and smooth in the middle, and finally only smooth muscle in the lower part.

The characteristic features of the esophagus are the combination of stratified squamous epithelium and the considerable thickness of the muscularis mucosae, much thicker than in other parts.

 

Figure 5: Stratified squamous epithelium of the esophagus

Stomach

The stomach is a muscular sack for the storage and digestion of food. The stomach can be divided into three regions:

  • fundus
  • body
  • pylorus

Each stomach region contains slightly different mucosa that reflects their different function.

At the junction with the esophagus, the stratified squamous epithelium of the esophagus abruptly changes to the simple columnar epithelium of the stomach. Simple columnar epithelium is recognizable by the shape and position of the nuclei. They are elongated and arranged in a neat, single row along the basement membrane.

 

Figure 6: Simple columnar epithelium of the stomach at very high magnification.

The stomach epithelium invaginates to form multiple gastric pits. At the bottom of each gastric pits lie gastric glands that reach deep into the lamina propria. Gastric glands produce stomach acid, pepsinogen, and mucus that are then secreted into gastric pits.

Figure 7: Gastric glands and gastric pits of gastric mucosa.

Small intestine

The small intestine is a long tube that extends from the stomach to the junction with the large intestine (a.k.a colon.) The major functions of the small intestine are digestion, secretion, and absorption. The small intestine is divided into three segments:

  • duodenum,
  • jejunum
  • ileum.

The mucosa of the small intestine has some adaptations to the functions it serves. It is heavily creased into the structures that increase the surface area where the nutrients are digested and absorbed. These adaptations include intestinal folds called the plicae circulares, villi, and microvilli.

The plicae circulares are folds of mucosa and submucosa that extend into the intestinal lumen. They encircle the entire intestine and are visible without a microscope, so they will not be discussed here.

Villi are microscopic fingerlike projections of the mucosa covered in simple columnar epithelium. Microvilli are tiny cytoplasmic extensions on the external surface of epithelial cells. They are barely recognizable under a light microscope as a striated layer on top of the epithelial cells and are called brush (striated) border.

Figure 8: Villi in the jejunum

All three segments of the small intestine are covered by simple columnar epithelium. Most of the cells are absorptive cells; interspersed among them is a small number of mucus-producing goblet cells that appear as tear shaped dots of a lighter color.

Figure 9: The arrows indicate Goblet cells in the small intestine.

Large intestine

The large intestine (a.k.a colon) connects the end of the ileum to the anal canal. In the large intestine, the intestinal content that arrived there from the small intestine is dehydrated and compacted into feces. The large intestine starts as a pouch called cecum and continues as the ascending, transverse, descending and sigmoid colon, followed by the rectum and anus.

The large intestine has the same four layers as other parts of the digestive tract. It is lined by simple columnar epithelium. The characteristic features of the large intestine are the lack of villi and the presence of the intestinal crypts (glands). Intestinal crypts are tubular glands, descending deep into the submucosa, with narrow openings to the surface. They are lined by the same simple columnar epithelium as the surface but have the abundance of mucus-producing goblet cells. Intestinal glands are visible in histology sections in transverse, longitudinal, or oblique planes in a variety of shapes.

Figure 10: Mucosa of the large intestine. Notice the abundance of lightly stained goblet cells within the intestinal crypts.

Lamina propria of the large intestine mucosa contains multiple nodules of lymphatic tissue that appear as darker stained spots.

Figure 11: Nodules of lymphatic tissue in the colon

Muscularis externa of the large intestine differs from other parts of the digestive tract. While the circular layer is the same, the longitudinal layer is arranged into bands of muscle called taeniae coli.

Region

Histological characteristics

Esophagus

Stratified squamous epithelium
Two muscularis externa layers: circular and longitudinal
Upper portion of muscularis externa is skeletal muscle
Thick muscularis mucosae

Stomach

Simple columnar epithelium
Three muscularis externa layers: oblique, circular, longitudinal
Gastric pits

Small Intestine

Simple columnar epithelium
Two muscularis externa layers: circular and longitudinal
Villi and microvilli
Small number of goblet cells

Large Intestine

Simple columnar epithelium
Two muscularis externa layers: circular and longitudinal
Lacks villi
Intestinal crypts (glands)
Abundance of goblet cells
Taeniae coli
Lymphatic nodules in lamina propria

 

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Histology Copyright © by Malgosia Wilk-Blaszczak. All Rights Reserved.

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